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Rep. Michael N. Castle, the moderate Delaware Republican who was expected to cruise to an easy win for the state’s open Senate seat, has been forced to spend money and resources to fend off a late surge from tea party-backed challenger Christine O’Donnell.

Mr. Castle is still expected to clinch the nomination in the Sept. 14 primary, and the primary fight is seen by political analysts as sharpening the low-key Castle campaign for the general election fight against Democrat Chris Coons, New Castle County executive.

Jessica Levinson, director of political reform for the Los Angeles-based Center for Governmental Studies, said the 2008 Democratic presidential primary is a classic example of how a party benefited from a long, hard-fought race because it helped party nominee Barack Obama defeat Mr. McCain.

“The race made both much better candidates,” she said. “It gave them a chance to react to negative publicity, get their sea legs and gain experience in running a serious campaign,” she said.

However, Ms. Levinson adds that the need to spend money also is a major concern in tough primaries, particularly when candidates face self-funded opponents. “In a bad economy, you don’t want to max out early,” she said.

Still, not everybody agrees that a hard, expensive fight for the party nomination is the best scenario.

“You always want to avoid a primary fight,” said Ben Tulchin of Tulchin Research, a San Francisco-based Democratic research and polling firm. “It’s a double whammy - damaging to the reputation and to your money in the bank.”